It is well known how to coat a metal strip, particularly a steel strip, with a layer of protecting metal, e.g. zinc, by making said metal strip pass through a bath of molten zinc.
In order to obtain the most uniform possible coating layer, it is further known to blow, onto both surfaces of the metal strip coming out of the bath, a jet of steam or gas under pressure (generally air) through a long and thin nozzle whose length is at least as the strip width.
It is also known that when said nozzle has a constant thickness, the gas or steam pressure at both ends of the nozzle decreases and has the behaviour shown in FIG. 1, wherein the ordinates denote the pressure (p) and the abscissaes the nozzle length (L).
It is further known that when said strip comes out of the molten zinc tank, the coating metal tends to deposit more on the strip edges than on the central area thereof, as in said two zones of the strip the cooling is quicker.
In order to remove the first mentioned inconvenience (decrease in the jet pressure from the centre towards the ends of the nozzle), a known nozzle was used having a larger width at its ends than at its center, in such a way, being the delivery of the nozzle higher at its two ends, is counterbalanced the pressure decrease shown in FIG. 1. A device suitable to embody effectively said characteristic is for instance claimed by the Italian Patent No. 896,562 of the same applicants.
Further, according to a subsequent patent still of the same applicant, two shutters were provided at the two ends of said elongated nozzle, said shutters defining exactly the width of the steam or gas blade according to the width of the metal strip to be coated. In this way, the mutual strike of the two opposed jets in the areas just adjacent the strip edges is avoided, improving so the critical conditions of pressure.
Said shutters have also the task of removing the noise caused by the strike of said opposed jets against each other. The distribution of the pressure along the nozzle after using said shutters is shown in FIG. 2, wherein the abscissaes denote the nozzle length (L) and the ordinates the pressure (p) along same.
When comparing said FIGS. 1 and 2, the advantages obtained by using the improvements shown in said two patents of the same applicants appear clearly.
However, it was noted that even by using a pressure diagram of the type shown in FIG. 2, it is not possible to remove the inconvenience of a larger thickness of the coating layer at the two side ends of the metal strip; such an inconvenience, as said above, depends on the higher cooling speed of the coating metal at the edges of the metal strip.